1. Field of the Invention
The invention is related to passive optical networks and more particularly to upstream scrambling systems for a passive optical network.
2. Description of Background Art
The use of passive optical networks (PONs) is increasing significantly as demand for additional speed and bandwidth in networks is increasing. The International Telecommunication Union (ITU) has published a standard addressing broadband optical access based upon PON. This standard, the ITU G.983.1 xe2x80x9cBroadband optical access systems based on Passive Optical Networks (PON)xe2x80x9d (October 1998), is incorporated by reference herein in its entirety. Another standard, the ITU-T I.432.1 xe2x80x9cB-ISDN user-network interfacexe2x80x94Physical layer specification: General Characteristicsxe2x80x9d (February 1999), sets forth the recommended standard for a layer 1 interface and is incorporated by reference herein in its entirety.
FIG. 1 is an illustration of a PON according to the preferred embodiment of the present invention. The PON includes an optical line termination unit (OLT) 102, an optical distribution network (ODN) 104, and one or more optical network units (ONUs) 106A, 106B. The ODN 104 offers one or more optical paths between one OLT 102 and one or more ONUs 106. Each optical path is defined between reference points S and R in a specific wavelength window. The two directions for optical transmission in the ODN are identified as (1) the downstream direction for signals traveling from the OLT 102 to the ONU(s) 106, and (2) the upstream direction for signals traveling from the ONU(s) 106 to the OLT 102.
In one example, the PON architecture in FIG. 1 is used to support asynchronous transfer mode (ATM) networking over PON. For ease of discussion, the present invention will be discussed with reference to ATM over a PON. However, it will be apparent to persons of ordinary skill in the art, that other protocols are supported by the present invention. In one embodiment, the ODN 104 is a optical fibre cable which can include conventional passive optical splitters in order to connect the OLT 102 to multiple ONUs 106 in order to share the capacity of the fiber. Because of the passive splitting, special actions are required with respect to privacy and security. Moreover, in the upstream direction a time division multiple access (TDMA) protocol is used.
The interface at the reference points S/R and R/S in FIG. 1 is defined as IFPON. This is a PON-specific interface that supports all the protocol elements necessary to allow the transmission between OLT 102 and ONUs 106 and is described in more detail in the ITU G.983.1 standard, for example. Point S corresponds to a point on the optical fibre just after the OLT 102 for downstream transmissions and just after the ONU 106 for upstream transmissions. Point R corresponds to a point on the optical figure just before the ONU 106 downstream transmissions and just before the OLT 102 for upstream transmissions.
The Optical Network Unit (ONU) 106 interfaces over the IFPON to the OLT 102. Together with the OLT 102, the ONU 106 is responsible for providing transparent ATM transport service.
In this architecture, the ATM transport protocols at an IFPON are described as consisting of Physical Media Dependent layer, Transmission Convergence layer, and ATM layer. This architecture is only intended to address the transport of ATM, further detail is contained in ITU Recommendation I.732 which is incorporated by reference herein in its entirety. The Physical Media Dependent layer would include the modulation schemes for both the upstream and downstream channels (they may be different). It may be possible for the specification to allow for more than one type of Physical Media Dependent layer in a single direction. The Transmission Convergence layer will be responsible for managing the distributed access to the upstream PON resource across the multiple ONUs 106. This will directly affect the resulting ATM quality of service (QoS). The ATM protocols should see no change in the way they operate over the PON. Within both the OLT 102 and the ONU 106, the functions performed at the ATM layer at both an OLT 102 and ONU 106 would include cell relaying.
The Optical Distribution Network 104 provides the optical transmission means from the OLT 102 towards the users and vice versa. It utilizes passive optical components.
FIG. 2 is a functional illustration of an optical line termination unit 102. The OLT 102 is connected to the switched networks via standardized interfaces, e.g., VB5.x, V5.x, NNI""s. At the distribution side, it presents optical accesses according to the agreed requirements, in terms of bit rate, access latency, etc. The OLT 102 includes three parts: the service port function 202; an ODN interface 204; and a multiplexor (MUX) 206 for virtual circuit (VC) grooming.
The MUX 206 provides VP connections between the service port function and the ODN interface and different VPs are assigned to different services at IFPON. Various information such as main contents, signalling, and OAM flows are exchanged by using VCs of the VP.
The ODN interface 204 handles inserting ATM cells into the downstream PON payload and extracting ATM cells from the upstream PON payload.
In general, the ODN 104 provides the optical transmission medium for the physical connection of the ONUs 106 to the OLT 102. Individual ODNs 104 may be combined and extended through the use of optical amplifiers as described in ITU Recommendation G.982, which is incorporated by reference herein in its entirety. However, the use of optical amplifiers are not necessary for the operation of the present invention.
The ODN 104 can include passive optical elements such as single-mode optical fibres and cables, optical fibre ribbons and ribbon cables, optical connectors, passive branching components, passive optical attenuators, and splices. More detailed information concerning passive optical components is described in ITU Recommendation G.671, which is incorporated by reference herein in its entirety. Additional information describing optical fibres and cable is described in ITU Recommendation G.652, which is incorporated by reference in its entirety.
In the context of the reference configuration illustrated in FIG. 1, FIG. 3 is an illustration of a physical configuration of an optical distribution network 104. As described above, the two directions for optical transmission in the ODN are (1) the downstream direction for signals travelling from the OLT 102 to the ONU(s) 106 and (2) the upstream direction for signals travelling from the ONU(s) 106 to the OLT 102.
Transmission in downstream and upstream directions can take place on the same fibre and components (duplex/diplex working) or on separate fibres and components (simplex working).
The ODN 104 offers one or more optical paths between one OLT 102 and one or more ONUs 106. Each optical path is defined between reference points in a specific wavelength window.
One aspect of PON communication as defined in the ITU G.983.1 standard is that ATM cells transmitted in the upstream direction (ONU 106 to OLT 102) are scrambled. However, this scrambling operation is flawed. The G.983.1 standard requires scrambling of the data in the cell. If the cell is not received, the cell is resent using the same scrambling operation such that the exact same sequence of bits are resent. There are many reasons why a transmitted cell is not received. One such reason is that the data pattern (e.g., a pattern including a long string of binary zeroes or ones) is such that the receiver does not recognize the cell. In such situations, simply resending the same data will not remedy the problem at the receiver, e.g., the OLT 102.
What is needed is a passive optical network system and method for enabling upstream data to be sent such that the receiver will recognize and receive the cell even if the data pattern of the originally sent cell cannot be recognized by the receiver.
The invention is a system and method for enabling an optical network unit (ONU 106) in a passive optical network to scramble data and send the scrambled data upstream to an optical line termination unit (OLT) 102. In passive optical networks the clocks in the OLT 102 and ONU 106 are synchronized by recovering the clock from the data signal. However, the clocks may drift when no data transitions occur on a long string of data. That is, a data sequence having a long string of consecutive binary zeroes or ones (CIDs, or Consecutive Identical Digits) may result in the receiver (OLT 102) drifting out of clock synchronization with the transmitter (ONU 106). In addition, the OLT may require data transitions to ensure proper adjusting of its receive threshold to compensate for ONUs on different lengths of fibre. As such, the receiver may drift out of its proper receive threshold setting if there are few transitions. In either circumstance, collectively called Loss of Synchronization, the data may not be received correctly by the receiver and the transmitter will need to resend the data, e.g., in an ATM cell. In the present invention, the transmitter will vary the seed used in the scrambling operation in such a way that the receiver will also know this seed. The use of a different seed per each transmission, even if the same scrambling function is used, significantly reduces the chances that a loss of synchronization will occur in the retransmitted cell.